Treatment of gases and vapors



Patented Dec. 7, 1943 2,336,097 TREATMENT or GASES AND vAPoRs Arthur John Lindsay Hutchinson, Los

Angeles,

Calif., assignor to The Fluor Corporation, Ltd., Los Angeles, Calif., a corporation of California Application November 26, 1940, Serial No. 367,191

(Cl. 19E- 8) 7 Claims.

This invention relates to an improved method and apparatus for the treatment of mixtures of gases and vapors to recover a desired condensate therefrom and has particular reference tothe recovery of gasoline from natural gas or vfrom analogous gaseous mixtures such as refinery still gases.

An object of the invention is to provide an absorption process and apparatus for the treatment of mixtures of gases and vapors to recover desired condehsates therefrom in which lower absorption temperatures may be maintained by the aid of waste heat rfrom the system. Another object of the invention is to provide heat economizers of such character that an increase of 25% ormore in the recovery of butane or other desired constituents is made possible in an yabsorption plant without any additional heat load on the plant.

A further object is to provide improvements in heat exchange in an absorption plant to substantially double the treating capacity of a given plant without additional equipment.

rWith these primary objects in view, the Vin- 'ention may be applied to an absorption system, such as is used for recovering condensates from natural gas, as shown in the diagrammatic sheets of drawings. Referring to Fig. 1 of the drawings, it may be seen that natural gas, for instance, containing the so-called wild gases consisting of methane, thane, and also propane, iso-butane, n. butane, iso-pentane, n. pentane, and perhaps portions of other hydrocarbon fractions o f higher boiling points, is passed through line I into any well known absorption tower II under any desired pressure which may be as high as 2000 pounds per square inch and generally ranges from V100 to 1500 pounds persquare inch.

fIn tower II the ascending gases are contacted with descending absorption oil supplied to tower `Il from an accumulator tank I2 through line I3 by the use of pump I4, which may be the usual steam pump, the supply of absorption oil being governed by a flow regulator I5 while its temperature is reduced to a desired degree by passage through a cooler I6 and a heat exchanger Il. The contacting of the gas with the cooled absorption oil under a predetermined pressure results in the abstraction by absorption of desired fractions While undesired fractions are Vented through line I8 controlled by pressure regulator The fat absorption Voil containing desired fractions is withdrawn 'from' tower` II through line 20 and passes, under a reduced pressure, into a vent tank 2| to release certain of the absorbed fractions which then .pass through line 22, controlied by pressure regulator 23, under a further reduced pressure into a stripper 24.

The remainder of the absorption oil is withdrawn from vent tank 2| through line 25, controlled by liquid level regulator 26, and picks up heat by passing through heat exchangers 2, 2t, and heater 29 before discharge into stripper 24. In stripper 24, the heated absorption oil is completely stripped of all absorbed constituents by open steam added through line 30 and the vapors and gases from line 22, the temperature .and pressure in stripper 24 being such that some of the heavier absorption oil will be carried off together with the released fractions through line 3| for recovery and lfurther uses. Since the still temperatures are such as to produce partial vaporization of a portion of the absorption oil, the vapor temperature is correspondingly high. Consequently, there is available a relatively high temperature level or potential which may be utilized to particular advantage for purposes of cooling or refrigeration, as will appear.

The lean or denuded absorption oil from stripper 24 passes through line 32, controlled by liquid level regulator 33, into accumulator l2, its temperature being reduced by passage through heat exchangers 28 and 2l.

The desired fractions from stripper 24 pass in vapor form through line 3l into an ammonia plant generally designated as 34 in Which the heat of the vapors is utilized for some of the purposes of this invention.

This is accomplished by passing the hot vapors from line 3l into an annular space, or condensing zone, 35 surrounding an ammonia still 36 whereby suiiicient heat is supplied to the ammonia liquor in still 3S to liberate the ammonia gas.

Thehydrocarbon condensate from Zone 35 is withdrawn, together with any gas, throughline 37 and passes through cooler 3S and water Ytrap 39 into a low pressure absorption tower V4l'which is supplied with cold absorption oil lfrom accumulator I2 by the use of line 4 I., pump 42, oW regulator 43 and cooler 44.

The reabsorption `in absorber 40 assures the removal of all absorption oiland desired fractions from the bull: of the remaining undesired "fractions which may then v:passthrough line 45 to any desired place of use.

. The -fat absorption oil from tower 40 is withdrawn through line 46, controlled by `liquid level regulator 41, by pump 43 and forced through heat exchanger 49 and heater 5I) into a fractionating tower 5I, wherein the absorption oil is closely fractionated to remove all desired fractions and make a clean separation between such fractions and the absorption oil.

This is done by the application of predetermined pressures and the use of live steam from line 52, or closed steam through coil 53, or both, and the stripped absorption oil passes through line 54, controlled by liquid level regulator 555, and heat exchanger 49 back into accumulator I2.

The vaporized fractions from fractionater 5I pass through line 55 and condenser 56 into a refiux condensate tank 51 whence a portion of the condensate is taken by pump 58 to be forced through a line 59, controlled by flow regulator 60, into the upper part of fractionater 5I, while any water may be abstracted through trap 6 I.

The remaining accumulating condensate from tank 51, in which .a reiiux supply is maintained by regulator 62, is taken by pump 63 and forced through line 64 Aand heat exchanger 65 into a stabilizer 56 wherein stabilization is effected to obtain a product having the desired vapor pressure and boiling range. Y

This is accomplished by maintaining a predetermined pressure in stabilizer 66 while supplying heat through a reboiling heater 61 so that all undesired fractions pass overhead through cooler 68 and condenser 69 to a reflux condensate tank 10, controlled by liquid level regulator 1I, whence a portion-may be returned to the top of stabilizer 66 by wayvof line 12 and flow regulator 13 while another portion is withdrawn through pipe 14. Any undesired gases are Withdrawn through line and pressure regulator 16 into discharge pipe 45 for further disposal.

The stabilized product is withdrawn from the bottom of stabilizer tower 66 and reboiler 61 through line 11 and its temperature is reduced toatmospheric temperature, or higher or lower, by` means of heat exchanger 65 and cooler 18 while on its way to storage 19.

' Merely by way of illustration and not of limitation, vone example of the pressures used through the absorption and stripping` and fractionating system heretofore described is tabulated:

' Pressure, pounds Absorption tower II 425 Vent tank ZI 225 Stripper 24 55 Absorber 46-; r52 Fractionator 5 I 55 Stabilizer 56 200 The temperatures in the various elements, as well as the above pressures, will be varied to meet conditions requiring a specification stabilized product and the above described system forms but the foundation for the present invention now to be more fully described for assuring that the capacity of the above described system can be doubled with an increased yield of desired and useful fractions. A

. In addition to the heat exchangers already mentioned the invention includes the use of the heat abstracted by the ammonia plant 34 from the overhead from stripper 24.

'I'he ammonia vapor from ammonia still 36 passes through line 88 into a condenser 8| where it is condensed. A small amount of liquid ammonia is returned to still 36 through line 82 to prevent water from passing'overhead from the still. The balance of the liquid condensed passes through line 83 and line 84 to be expanded through valve 95 into gas cooler 86. On expansion, part of the liquid ammonia evaporates, dropping the temperature of the balance. This cold liquid then boils around the tubes of the heat exchanger, extracting heat from the incoming gas. The vapors from this gas cooler pass into ammonia absorber 81 where they are absorbed in cooled weak ammonia liquor brought from still 36 by line 89. Another stream of liquid ammonia. is taken from line 83 through line 90 and expanded through valve 9| into oil chiller I1 where, by the same process described for the gas, the oil is chilled passing through line I3 into absorber II. The ammonia vapors generated in oil chiller I1 pass through line 96 into ammonia absorber 81.

The water charged with the ammonia from .ammonia absorber 81 is forced by pump 88 through line 92 into ammonia still 36 where the ammonia is recovered for use in the cycle. A heat exchanger 93 and a cooler 94 serve to bring thetemperature of the weak ammonia liquor down to the desired degree.

In this manner, by the use of heat normally wasted from stripper 24, not only is refrigera-V tion provided in tower II but such refrigeration is provided at a much lower operating cost than is possible under present operations because no additional heat is applied to the system from any outside source. This also reflects a saving in heat abstracted from the vapors from stripper 24 in place of being abstracted by water cooling in a cooling tower or in some other manner. This results in very much increasing the absorption efciency of tower I I due to the much higher selectivity of absorption obtained at a lower temperature and the resulting increased absorption of the desired fractions; consequently, -the capacity of theremainder of the system will be increased substantially in proportion to the increased absorption eiiiciency of tower II.

While normally there is sufficient heat available in the vapors from stripper 24 to operate ammonia still 36, as described, if the available heat is insufficient for this purpose, further economyr-may be eifected in the system by utilizing l exhaust steam from pumps I4 and 42 or any other pumps in the system.

As illustrative of the beneficial eiects of the described ammonia refrigeration from waste heat and comparing the invention with a similar plant in which the` vapors from stripper 24 are passed directly through an ordinary condenser 38, there is found to be an increase in capacity of from 60% to 85% without anyrincreaseof oil circulation in the main absorber I I. At the same time, the veciency of the main absorber is greatly increased dueto a drop in temperature from F. to 55 F. by reason of the ammonia refrigeration. Y'

This is accomplished without any increase of other plant equipment except in stabilizer 66 which increased in heatV load due to the larger volume of material handled. However, the heat load on stabilizer 66 is a relatively small part of the total heat loadthrough the described system. andthe use of stabilizer 66 is optional with the present invention, particularly in view of present-day renery practice in which large volumes of unstabilized gasoline are now customarily'handled. y f

VDepending on the available refrigeration from ammonia'still'34, a portion of theliquid ammonia may be passed through header 95 into al chiller96 surrounding pipe 31 to materially reduce thetemperature of condensate and gases 2,336,097 entering reabsorber '40 and the 'expanded ammonia gas will pass through line 91 into header 98 and thence to ammonia absorber 81.

Similarly, further liquid ammonia can pass from header 95 into line 99 and around cooler 44 to chill the absorption oil entering reabsorber 40, the expanded ammonia then passing through line into header 98. Inthe event that cooler 44 is used as a refrigerator with ammonia gas, a precooler |0| may be additionally used.

In the event that it is desired to make a high recovery ofvery volatile fractions, header 95 may terminate in a chiller 69 to which vapors precooled in cooler 68 are supplied,` and the eX- ,panded ammonia is thence returned through pipe |02 to header 98 for recovery and reuse.

It will be understood that While the use of ammonia has been heretofore described as giving the-high efficiency in the system by use of Waste heat, the term ammonia is merely illustrative and the invention comprehends the use of any mixture of liquids and volatile absorbed refrigerants known in the art. For instance, mixtures of certain absorption oils with either propane, butane, ethylene or propylene are quite suitable, as well as other similar mixtures.

Referring to Fig. 2, in which like numerals indicate parts similar to Fig. 1, an improved result is likewise obtained by using the ammonia plant 34 to abtract the waste heat of the hot oil bottoms from fractionater `In -this case, the heat exchanger 49 is dispensed with and the hot stripped absorption oil passes through line 54 into the annular heating zone 35 in which it imparts its Waste heat to the ammonia liquor in still 36, the ammonia from which is utilized to obtain the refrigerating effect previously described. The thus cooled absorption oil then passes through line |03 back to accumulator I2 for reuse, or all or a portion thereof may pass through line |04 into line 32 to have further heat abstracted in heat exchanger 21.

According to this disposition of ammonia plant 34, the hot vapors from stripper 24 will pass through line 3| and heat exchanger |04 wherein they lose heat to the cold unstripped absorption oil passing through line 46 to fractionater 5|.

At times it may be desirable to use the dispositions of Fig. 1 and Fig. 2 in a single system provided the waste available heat in either is insufcient to provide suflicient refrigeration for all the purposes described herein, in which event the required connections for the liquid ammonia may be made very readily, the primary factor being the need for assuring the requisite lowering of the temperature of absorption at the main absorber |I.

While heat exchangers l1 and 86 are shown in the drawings exterior to main absorber it will be understood that these may be interior of tower at the top and bottom to assure the desired effects.

I claim as my invention:

1. The process of treating gases and vapors which comprises: passing a mixture of hydrocarbon gases and vapors under superatmospheric pressure through lean higher boiling absorption oil in a first absorption zone to absorb certain fractions therefrom, thereafter reducing the pressure and increasing the temperature of the resulting rich absorption oil to release absorbed hydrocarbon fractions in vapor form together with some absorption oil, passing said heated released vapors in indirect heat exchanging rela- 'tion with refrigerant liquor' to release gaseous refrigerant therefrom and to partially condense said released` hydrocarbon vapors, reabsorbing the resulting condensate in lean reabsorption cil in a second absorption Zone and at a reduced temperature, fractionatingv the resulting rich reabsorption oil at an increased temperature to completely vaporize and separate the hydrocarbons of said condensate, passing the thus fractionated lean reabsorption oil in heat exchanging relationship with the cooler rich unfractionated reabsorption oil, and condensing and then expanding the released gaseous refrigerant to pass in heat exchanging relationship with the rst mentionedlean absorption oil and said mixture of gases and vapors prior to their initial contact in the system.

2. In a process for absorbing hydrocarbon fractions from gases in which a stream of gas is passed through higher boiling absorption oil wherein certain fractions are absorbed, the absorption oil is then heated to separate absorbed fractions in vapor form together with some absorption oil, and the separated fractions are recovered, that combination of steps which comprises: passing the hot separated vapors at substantially their temperature of separation from the unvaporized absorption oil in indirect heat exchange relationship with refrigerant liquor to release refrigerating gas therefrom, liquefying said refrigerating gas, expanding the liquefied refrigerating gas in indirect heat exchange relationship with said absorption oil to cause Inaterial reduction in the temperature of absorption of said hydrocarbon fractions, then reabsorbing the thus utilized refrigerating gas in cooled liquor from which it was released.

3. In a process for absorbing hydrocarbon fractions from gases in which a stream of gas is passed through higher boiling absorption oil wherein certain fractions are absorbed, the absorption oil is then heated to separate absorbed fractions in vapor form together with some absorption oil, and the separated fractions are recovered, that combination of steps which comprises: passing the hot separated vapors at substantially their temperature of separation from the unvaporized absorption oil in indirect heat exchange relationship with refrigerant liquor to release refrigerating gas therefrom, liquefying said refrigerating gas, expanding the liqueed refrigerating gas in indirect heat exchange relationship with said hydrocarbon fractions to materially reduce their temperature during absorption in said absorption oil, then reabsorbing the thus utilized refrigerating gas in cooled liquor from which it was released.

4. In a process for absorbing hydrocarbon fractions from gases in which a stream of gas is passed through higher boiling absorption oil wherein certain fractions are absorbed, the absorption oil is then heated to separate absorbed fractions in vapor form together with some absorption oil, and the separated vapors are recovered, that combination of steps which comprises: passing the hot separated vapors at substantially their temperature of separation from the unvaporized absorption oil in indirect heat exchange relationship with refrigerant liquor to release refrigerating gas therefrom, liquefying said refrigerating gas, expanding the liquefied refrigerating gas in indirect heat exchange relationship with said absorption oil and said hydrocarbon fractions to cause material reduction in the temperature of absorption during Contact,

'then reabsorbing thethus utilized refrigerating `gas in cooled liquor from which it was released.

5. In a process for absorbinghydrocarbon fractions from gases in which a stream of gas is passed through higher boiling absorption oil wherein certain fractions are absorbed, the absorption oil is then heated to separate absorbed fractions in vapor form together with some absorption oil, and the separated vapors arel recovered, that combination of steps which comprises: passing the hotrseparated vapors at substantially their temperature of separation from the unvaporized absorption oil in indirect heat exchange relationship with refrigerant liquor to release refrigerating gas therefrom, liquefying said refrigerating gas, simultaneously expanding the liquefied refrigerating gas in two separate streams, one of which passes in indirect heat exchange with said hydrocarbon fractions and the other of which passes in indirect heat exchange with said absorption oil prior to their contact, to effect material reduction in their temperature during absorption and material increase in the percentage of hydrocarbon fractions absorbed, then reabsorbing the thus utilized refrigerating -gas in cooled liquor from which it was released.

6. Apparatus for absorbing hydrocarbon fractions from gases comprising in combination: a main absorption tower, a heated stripper, a second absorption tower and a fractionater, means to pass a continuous supply of hydrocarbon gases into the main tower, means to pass a continuous supply of absorption oil into the main tower, a .connection to pass the absorption oil from the main tower into the stripper wherein absorbed fractions are liberated together with some of the absorption oil, means including a refrigerant still to abstract heat from said liberated fractions and to vaporize refrigerating gas therein, means to 'pass the `cooled fractions into said second absorprtion tower' into contact with further absorption oil therein, a pipe from said second tower to 'said fractionater to pass said further absorption oil, a withdrawal line from said fractionater in 'indirect heat exchange with said pipe, refrigerat- ,ing gas connections from said refrigerant still in 'indirect heat exchange with-said continuous gas .supply and said continuous absorption oil supply including'V means forco'ndensing and expanding the'refri'gerating gas yfro'm'said still, and means 'forabsorbing said expanded refrigerating gas in cooled liquor from said still and returning the 'mixture to said still. Y

'7. In a process for absorbing hydrocarbons from gases in which a stream of gas is passed through afstream of higher boiling absorption oil wherein certain fractions are absorbed, the absorption oil .is heated in a stripping rone to separate absorbed fractions in Vapor form together with some abfsorption oil, and the separated fractions are recovered, that combination of steps which comprises: introducing steam to said stripping zone `ARTHUR JOI-1N LINDSAY HUTCHINSON. 

